The short answer is you can't.
Not that there isn't any connectivity between lemmy and mastodon.
Mastodon is purely about following and reading feeds of people's "microblogs". Lemmy is purely about communities and the posts and their comments in those communities.
Right now (as this stuff may very well change) you can't follow people on lemmy, only communities. However, lemmy does "federate" its communities to mastodon (and other microblog platforms) in a way that makes them look like "people". This means that mastodon people can follow a community and post to a community by just treating the community like any other person/user. So mastodon people can and do pop into our communities by posting or, more commonly, commenting on posts. And once mastodon people make contact, we can talk to each other (ie, we can reply to their posts/comments, and they can read them etc).
I don't know where the idea of a lemmy feature to allow following users (here or on mastodon) is up to (you'd have to check the matrix rooms or github issues I guess), though it's certainly plausible. Otherwise, I can see lemmy just focusing on getting this community based platform right so that it works well at scale.
And, if you are not aware, there's another platform with communities called kbin that also integrates with the microblog platforms. You should check it out.